There is a seminal new study of drought in the western United States that extends the period of assessment back to 800 A.D [and thanks to Connie Woodhouse for providing me a copy of their paper]. The paper is
Meko, D., C. A. Woodhouse, C. A. Baisan, T. Knight, J. J. Lukas, M. K. Hughes, and M. W. Salzer (2007), Medieval drought in the upper Colorado River Basin, Geophys. Res. Lett., 34, L10705, doi:10.1029/2007GL029988.
The abstract reads
“New tree-ring records of ring-width from remnant preserved wood are analyzed to extend the record of reconstructed annualflows of the Colorado River at Lee Ferry into the Medieval Climate Anomaly, when epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western North America. The most extreme low-frequency feature of the new reconstruction, covering A.D. 762-2005, is a hydrologic drought in the mid-1100s. The drought is characterized by a decrease of more than 15% in mean annualflow averaged over 25 years, and by the absence of high annual flows over a longer period of about six decades. The drought is consistent in timing with dry conditions inferred from tree-ring data in the Great Basin and Colorado Plateau, but regional differences in intensity emphasize the importance of basin-specific paleoclimatic data in quantifying likely effects of drought on water supply.”
Figure 2 from that paper provides must be considered one of the most important illustrations of the natural variability of the climate system as well as the observation that the climate is never static.
Figure caption from Meko et al2008: Time series plot of 25-year running mean of reconstructed flows of the Colorado River at Lee Ferry . Flows are plotted as percentage of the 1906–2004 mean of observed naturalflows (18.53 billion cubic meters, or 15.03 million acre-ft). Confidence intervalderived from 0.10 and 0.90 probability points of ensemble of 1000 noise-added reconstructions. Horizontal dashed line is lowest25-year running mean of observed flows (1953–1977). [the Colorado River, of course, is a major source of water for much of the southwest United States].
What this figure tells us include that
- periods of drought occurred that were longer and more severe in the pre-historical time period
- the climate of this region is never static but varies significantly over time.
One obvious conclusion is that regardless of how humans are altering the climate system, the natural variations are significantly larger that stated in the 2007 IPCC assessment. This conclusions adds significant new support for the paper
Rial, J., R.A. Pielke Sr., M. Beniston, M. Claussen, J. Canadell, P. Cox, H. Held, N. de Noblet-Ducoudre, R. Prinn, J. Reynolds, and J.D. Salas, 2004: Nonlinearities, feedbacks and critical thresholds within the Earth’s climate system. Climatic Change, 65, 11-38,
where we report that
“The Earth’s climate system is highly nonlinear: inputs and outputs are not proportional,
change is often episodic and abrupt, rather than slow and gradual, and multiple equilibria are the norm”
“It is imperative that the Earth’s climate system research community embraces this nonlinear paradigm if we are to move forward in the assessment of the human influence on climate.”
The significance of these observational findings is that the regional multi-decadal predictions based on the current generation of IPCC models should not be used by the impacts and policy making communities. The IPCC models fail to skillfully predict climate features such as drought, as exemplified in the figure from the Meko et al paper. A more appropriate procedure for the impacts and policy communities would be to use historical and paleoclimate data to explore the consequences to society and the environment in the coming decades, if these extreme climate conditions were to reoccur. In terms of how humans are altering the climate system, we do not know if we are making such extremes more of less likely (despite scientifically flawed claims to the contrary; e.g. see and see), but we do know that such extreme weather conditions occurred in the past. Therefore, prudent policy management would plan for their eventual reoccurrence.